Interpretation of the impedance of the silicon electrolyte interface is particularly complicated under conditions in which anodic silicon dissolution occurs. In the present work the frequency-resolved potential modulated microwave reflectivity (PMMR) technique has been combined with electrochemical impedance spectroscopy (EIS) to study the behaviour of low-doped p-type silicon in buffered ammonium fluoride. The EIS response is determined by the total impedance of the system, whereas the PMMR response originates from modulation of the hole density in the silicon space charge region. This difference has led to the development of a novel approach to deconvolution of the interfacial impedance. It involves multiplication of the PMMR and EIS responses to give the space charge impedance. Particular attention is focussed on accumulation conditions in the electropolishing region, where the surface of the silicon is covered by a duplex oxide film. The impedance of the accumulation layer is obtained using the new approach, and the oxide impedance is then derived. Analysis of the PMMR results provides convincing evidence that the hole mobility in the accumulation layer is more than an order of magnitude smaller than in the bulk.